The present invention relates to novel means for combating insect pests, and in particular for combating aphids.
Insect pests constitute one of the main preoccupations in agriculture. Besides the damage produced by the insects themselves, attack by these insects very often promotes transmission and infection of the plants with bacterial, viral or fungal diseases.
Among pests, aphids (also called greenfly) are the most common. There are more than 4000 species of them, the most widespread of which are Myzus persicae and Aphis gossypii. 
Each species has a specific life cycle and a set of preferred hosts. Aphids are extremely dynamic organisms, which adapt rapidly to environmental conditions. This rapid adaptation is essentially due to the various reproductive strategies developed by aphids. Depending on the climatic conditions, aphids reproduce sexually or asexually and are oviparous or viviparous. They may or may not be winged, thus facilitating their passage from one plant to another. Due to this great ability to adapt and to reproduce, complete infestation of a crop in the open field and a fortiori in a greenhouse is extremely rapid. A single individual gives birth to between 40 and 100 larvae.
The cotton or melon aphid Aphis gossypii is present in most regions of the world, with the exception of the most northern. It is capable of performing a complete developmental cycle and of reproducing in less than a week. Because of its abilities to adapt and to reproduce, a large number of generations can be produced in a season, whatever the climatic conditions thereof. The melon aphid has a broad spectrum of hosts (approximately 700 cultivated or wild plants, including about 50 in France). Among these, the most sensitive are the Cucurbitaceae, including, for example, the melon, the courgette and the cucumber, the Malvaceae, such as the cotton plant or the hibiscus, and, to a lesser extent, the Solanaceae and the Rutaceae, such as citrus trees.
The melon aphid mainly colonizes the lower face of the leaves, the buds and the young shoots. By taking the nutritive elements thereof from the phloem, the aphid diverts the plant's resources and weakens it. The colonized tissues become chlorotic, the leaves roll up on themselves and the photosynthetic yield decreases. The aphid secretes a highly sugar-rich honeydew that serves as a substrate for saprophytic fungi, such as sooty mold, which deposits a black veil over the leaf, further reducing the photosynthetic capacities of the plant and causing a substantial commercial depreciation of the fruit and vegetables affected. In addition, the aphid is a vector for many viruses that it introduces directly into the phloem of the plant when it stabs the vessels.
Chemical control is currently still the most widespread technique. However, it has many drawbacks. The products used frequently have a broad spectrum of action and destroy beneficial insects at the same time as the aphids. The risks of polluting the environment are also considerable: aphicides are in fact among the most toxic products for humans, useful fauna and the environment (Recueil des effets non intentionnels des produits phytosanitaires [Collection of unintentional effects of plant protection products], Acta, 1998, p 256, Berger and Van Holst, 2001, Environ. Sci. Pollut. Res. Int., 8, 109-112). Moreover, the aphid Aphis gossypii has, in certain regions, developed resistances to the chemical compounds used, such as organophosphates, carbamates, pyrethroids and organochlorinated products (Larry et al., The Journal of Cotton Science, 5, 22-29, 2201; Delorme, Pesticide Science, 49, 90-96, 1997).
Biological control consists in using the natural predators and parasites of Aphis gossypii, for instance ladybugs, certain Hemiptera or pathogenic fungi. It can, however, only be used for greenhouse crops.
Another approach is based on the search for varieties that are naturally resistant to aphids, and on their use for varietal improvement and the creation of hybrids.
Aphid-resistant varieties have thus been found in particular in wheat, the apple tree, fodder pea, lettuce, tomato, etc.
In the melon (Cucumis melo), the existence of a dominant locus that confers resistance to the aphid Aphis gossypii has been discovered in melon lines originating from the Far East or from India. This locus, which was called Ag (for Aphis gossypii resistance) or Vat (for virus aphid transmission resistance), confers a double resistance phenotype: resistance to infestation of the plant with Aphis gossypii and resistance to transmission, by this aphid, of the viruses for which it is the vector (Kishaba et al., J. Econ. Entomol., 64, 935-937, 1971; Bohn et al., J. Amer. Soc. Hort. Science, 98, 37-40, 1973; Lecoq et al., Phytopathology, 69, 1223-1225, 1979; Pitrat and Lecoq, Phytopathology, 70, 958-961, 1980).
The resistance-promoting Vat locus has been introduced, by crossing, into various commercially available varieties of melon; however, the creation of aphid-resistant melon varieties by means of the usual techniques for varietal improvement remains lengthy and expensive.